In injection molding, molds are held and clamped in a mold press for cycling of the molding unit by closing and opening the mold halves. When the mold halves are brought together, they can form one mold cavity in single cavity molds, and more than one cavity in multi-cavity molds. During molding cycles, the mold halves are aligned with each other by leader guide pins on one of the mold halves entering aligned bushings in the other mold half. Typically, the mold halves are vertically aligned with the cavity or upper mold half provided with a lower cavity plate in which the leader pins are mounted and the core or lower mold half having an upper plate in which the bushings are mounted. Generally, four leader pins are mounted at the corners of the lower plate of the cavity half and four bushings are correspondingly located at the four corners of the upper plate of the core half. The pins are aligned with corresponding bushings and are sized to provide a slight clearance, generally on the order of approximately 0.001 inches, between the pins and their corresponding bushings. However, during mold fitting and assembly, there will usually be some slight misalignment between the mold halves. Thus, as the molding unit is cycled, the leader pins cooperate with the bushings to provide for a higher degree of alignment between the mold halves relative to each other as they are brought together for forming of a molded article and, during mold construction, to correct for any misalignment that has occurred such as due to misalignment of press platens.
In a growing number of molds, hot runner molding has become increasingly common to improve the efficiency of the injection molding process. When hot runners are used, the material in the sprue and runners, instead of being hardened and removed from the mold along with molded articles, is kept hot by means of the heating elements built into the mold and remains in the mold between shots. In this manner, the plastic molding material in the sprue and runners in a given shot remains hot enough and soft enough to be pushed into the mold cavity on the next shot and become a molded article. As the charge of heated plastic molding material is injected through the upper stationary cavity half of the mold, this half is typically termed the "hot half" while the core is referred to as the "cold half" providing a temperature differential between the two during molding operations.
In addition to the usage of the hot runner molding, the development of engineered thermoplastics used as molding material which are cycled at increasingly higher temperatures can produce relatively high temperature differentials between the cavity and core halves of the mold. As the lower plate of the cavity half of the mold is hotter than the upper plate of the core half, the amount of thermal expansion the cavity half will undergo will be greater than that in the core half. This causes the leader pins to move a greater extent than the bushings from their original positions. As the original clearance between the pins and bushings is relatively slight, any thermal expansion can cause interference therebetween. Interference between the pins and bushings caused by such thermal expansion generally leads to faster wearing of the pin and bushing contacting surfaces, such as by galling and scoring surface damage, over that encountered if thermal expansion was not a problem. Such wear of the leader pins and bushings can progress to the point to where it impairs their ability to perform their alignment function for the mold halves. In extreme situations, scoring of the pins and bushings can cause the mold halves to lock up as the pins and bushings essentially become welded to each other. As is apparent from the above, there are two primary sources of leader pin to bushing wear. One is where the pins and associated bushings are correcting misalignment by moving one mold half into proper registration with the other mold half. The other is when unequal thermal expansion is occurring between the mold halves causing the leader pins and their bushings to move off of their coinciding center points producing interference between the mating components.
Other bushings that need to be replaced and require a tearing apart of the mold to replace them are the bushings that are mounted in an ejector assembly having ejector pins that push out and eject the mold article from the mold cavity. The ejector assembly has stationary pins and ejector plate bushings which slide on the pins with cycling of the mold. Thus, bushings and/or pins are subject to wear and the misalignment problems described herein, and require a tearing apart of the mold to demount the old worn bushings and to replace them with new bushings. Hereinafter, (unless specified otherwise) the term "bushings" is used generically for the bushings in either mold or die half or in the ejector plate; and the term "pins" is used generally for mold half leader pins or the pins which guide the ejector plate.
The pins and/or bushings can be provided with a lubricant so that the pin more readily slides into and out of the bushing during a molding cycle despite the close tolerances therebetween. However, in many plastic molding applications, it is necessary to mold plastic parts in a production process that is free of oil, grease, or other liquid lubrication. This type of lubrication can migrate to the molding surface and ultimately to the molded part. In addition, the molded part may contact the leader pin upon ejection from the mold. Typical examples of where these are unacceptable occurrences would be plastic parts for medical applications, parts for the food packaging industry, electronic parts, and clear acrylic parts for lenses or displays. Moreover, where engineered thermoplastics are run at very high temperatures, most common forms of lubrication tend to break down so that they are no longer effective. Finally, even in applications allowing for use of lubrication, failure to maintain a proper amount of lubrication can lead to premature wear. As in apparent, in these "dry" applications or where the lubricants used to fail, the pins and bushings will be more quickly subject to wear and damage.
To run molds in proper alignment, worn pins and bushings must be removed from their associated mold plates and replaced with new components. Removal of damaged leader pins and bushings is a cumbersome and time consuming process. First, the molding unit must be shut down and the mold must be removed from the press. The bushings and pins are typically press or force fitted into bores in their respective mold halves. The mold is then disassembled with the pins and bushings then being pounded out of their respective mold or ejector plates and replaced with new pins and bushings, which are force fitted into the respective bores in the mold halves. The mold is then reassembled and installed back into the press until the pins and bushings once more need to be changed over. As is apparent, the above process leads to significant downtime reducing production efficiency which is undesirable.
To reduce the amount of downtime, molders sometimes attempt to avoid the significant time associated with the required disassembly of the plates by turning down the leader pins such as by 0.005 inches or opening the bushings a similar amount so that they do not make contact during molding cycles. When this occurs, the leader pins and bushings no longer serve their aligning function and another alignment mechanism is required on the mold halves, such as a side lock utilizing parallel surfaces which do not interfere when unequal thermal expansion occurs. Thus, there is a need for guiding mechanisms for mold halves which can be used in a wide variety of applications and run for a greater number of cycles. In addition, it is desirable for the guiding mechanism to be easily replaceable, such as if it is worn, without having to remove the mold from the press and then disassemble the mold.
It is desirable to retrofit existing molds without having to do any machining thereof when installing new, easily replaceable bushings that can be replaced without having to remove the mold from the press. That is, it is undesirable that the molds would have to be re-machined to accept replaceable bushings as this would result in additional down-time and labor costs. Further, machining of molds at the parting line leaves areas for flashing or collection of unwanted material along the parting line. Additionally, it is desirable to provide, easily replaceable bushing inserts for current mold designs that do not require the molds to be custom-made or custom designed for a special design of a bushing.